In the electronics industry, conductive metal wire is used in a variety of devices, such as semiconductor devices, to connect contact points on the device to other contact points. The most commonly used materials for wire bonding are gold and aluminum, although copper and silver are also used at times depending on the application. A wire bond is formed by attaching a length of wire between two contact locations. In order to form the attachment, various devices are used to sever and bond (e.g., melt) the wire ends to the contact location. Some of the most common devices used to sever and melt the wire are thermocompression (T/C), thermosonic (T/S) or ultrasonic (U/S) devices. The wire is typically formed with a generally parabolic or elliptical shape and is, thus, referred to as a wire “loop”.
Two well known techniques for bonding a wire to contact locations of an electronic device are ball bonding and wedge bonding. Ball bonding is generally the preferred technique, particularly in the semiconductor industry in which more than 90 percent of all semiconductor devices are manufactured using ball bonding.
Ball bonding apparatuses include a bond head carrying a wire bonding tool such as a capillary. A capillary is an elongated, tubular structure and has an axial passage through which a length of wire is fed for bonding by the bonding apparatus. Ball bonding apparatuses also typically include an electronic flame-off (EFO) wand that, when fired, supplies a spark that melts an end portion of the wire extending from the capillary. As the molten end portion of the wire solidifies, surface tension forms the end portion into a substantially spherical shape. The spherically shaped portion of the wire formed by the EFO wand is referred to as a “free-air ball”. The free-air ball is bonded to one of the contact points on the semiconductor device or substrate by plastic deformation of the ball onto the contact.
Referring to FIG. 1, there is shown a conventional capillary 10. The capillary 10 includes an elongated shaft 12 having a substantially cylindrical portion 14 and a conical portion 16. As shown, the capillary 10 defines an axial passage 18 extending through the capillary for passage of a wire to be bonded by a wire bonding apparatus. The axial passage 18 is located substantially concentric with the centerline of the capillary 10. The capillary 10 also includes a working tip 20 extending from conical portion 16 of the shaft 12 and located at a terminal end of the capillary 10. The working tip 20 of the capillary 10 is adapted to form wire bonds through plastic deformation and interfacial interaction at contact locations, for example, on a substrate surface. The working tips of known capillaries vary in configuration. An example configuration for the working tip of a capillary for a wire bonding apparatus is described in U.S. Pat. No. 6,715,658.
As shown in FIG. 1, the conical portion 16 of the capillary shaft 12 widens from the working tip 20 at an angle, which is sometimes referred to as the cone angle for the capillary 10. The cylindrical portion 14 of the capillary shaft 12 is engaged by a transducer (not shown) adjacent a terminal end 22 of the capillary 10 opposite the working tip 20. The transducer vibrates the capillary shaft 12 to supply ultrasonic energy at the working tip 20 of the capillary 10. The ultrasonic energy supplied by the transducer facilitates the above-described plastic deformation and interfacial interaction between the wire and the contact points at a bond site location. As shown in FIG. 1, the diameter of shaft 12 of the prior art capillary 10 remains substantially constant in the cylindrical portion 14 from the terminal transducer-engaging end of the shaft 12 to the intersection between the cylindrical portion 14 and the conical portion 16.
It is desirable that the diameter of the free-air ball formed at the end of the wire be aligned as closely as possible with the centerline of a capillary. Concentricity between the free-air ball and the capillary centerline is desirable for ensuring accurate placement of the wire bond with respect to a targeted contact location. It would be desirable for the EFO wand to be located in substantial alignment with the centerline of the capillary of a wire bonding apparatus. Such an arrangement would provide the greatest probability of concentricity between the resulting free-air ball formed at the end of the wire by the EFO wand and the capillary centerline.
Clearance between the capillary and the EFO wand is typically provided, however, to provide access for the working end of the capillary to the contact locations on a substrate surface. Accordingly, the EFO wand cannot be concentrically aligned with the wire diameter and, instead, must be located at a distance from the centerline of the associated capillary. As a result, the spark from the EFO wand is directed to the terminal end of the wire along a path that is oblique with respect to the capillary centerline.